What are the factors you consider for transaction fee spikes other than unexpected network traffic? Since you mentioned privacy based CEXs with btc fees around $5-$20 in the first point which doesn't sound like a normal single input-output tx fee considering the recent mempool situation, I'm curious what kind of privacy is that, especially how a user gets tx inputs for a single withdrawal (is it possible to get consolidated inputs from other users' exchange wallets at times, or all inputs from one centralized wallet).
As far as I know, the higher the level of privacy you offer, the higher the fees are and I think taking a percentage fee is fairer in this case.